1) Field of the Invention
The present invention pertains to systems and methods involved in radio communication systems and, more particularly, to a system and method for controlling antenna diversity.
2) Discussion of Related Art
The cellular telephone industry has made phenomenal strides in commercial operations in the United States as well as the rest of the world. Growth in major metropolitan areas has far exceeded expectations and is rapidly outstripping system capacity. If this trend continues, the effects of this industry's growth will soon reach even the smallest markets. Innovative solutions are required to meet these increasing capacity needs as well as maintain high quality service and avoid rising prices.
FIG. 1 illustrates an example of a conventional cellular radio communication system 100. The radio communication system 100 includes a plurality of radio base stations 170a–n connected to a plurality of corresponding antennas 130a–n. The radio base stations 170a–n in conjunction with the antennas 130a–n communicate with a plurality of mobile terminals (e.g. terminals 120a, 120b and 120m) within a plurality of cells 110a–n. Communication from a base station to a mobile terminal is referred to as the downlink, whereas communication from a mobile terminal to the base station is referred to as the uplink.
The base stations are connected to a mobile telephone switching office (MSC) 150. Among other tasks, the MSC 150 coordinates the activities of the base stations, such as during the handoff of a mobile terminal from one cell to another. The MSC 150, in turn, can be connected to a public switched telephone network 160, which services various communication devices 180a, 180b and 180c. 
A common problem which occurs in cellular radio communication systems is the loss of information in the uplink and downlink signals as a result of multi-path fading. Multi-path fading occurs when the transmitted signal travels along several paths between the base station and the intended receiver. When the differences in the path lengths between the base station and the mobile terminal are relatively small, the multiple signal images arrive at almost the same time. The images add either constructively or destructively, giving rise to fading, which can have a Rayleigh distribution. When the path lengths are relatively large, the transmission medium is considered time dispersive, and the added images can be viewed as echoes of the transmitted signal, giving rise to intersymbol interference (ISI).
Fading can be mitigated by using multiple receive antennas and employing some form of diversity combining, such as selective combining, equal gain combining, or maximal-ratio combining. Diversity takes advantage of the fact that the fading on the different antennas is not the same, so that when one antenna has a faded signal, chances are the other antenna does not. ISI multi-path time dispersion can be mitigated by some form of equalization, such as linear equalization, decision feedback equalization, or maximum likelihood sequence estimation (MLSE).
Interference can also degrade the signals transmitted between a base station and mobile terminals. For instance, a desired communication channel between a base station and a mobile terminal in a given cell can be degraded by the transmissions of other mobile terminals within the given cell or within neighboring cells. Other base stations or RF-propagating entities operating in the same frequency band can also create interference (e.g., through “co-channel” or “adjacent channel” interference in systems).
Frequency re-use can be used to, among other things, mitigate interference by placing interfering cells as far from each other as possible. Power control can also be used to reduce the interference by ensuring that transmitters communicate at minimal effective levels of power. Such power control techniques are especially prevalent in code-division multiple access (CDMA) systems, due to the reception of information in a single frequency channel at each base station.
The performance of individual radio links as well as the overall system is improved by antenna diversity in the mobile station and/or base station receiver. Having multiple antennas, and processing the information accordingly, helps combat fading and makes the communication link more robust. The price of introducing antenna diversity is the extra radio and base band processing resulting from having more than one signal to demodulate. This price in a mobile station has often been judged to be too large in terms of the increased space, manufacturing costs, and power consumption due to the extra components required.
Currently, one cellular system uses antenna diversity in the mobile station. Specifically, in the Japanese system PDC this form of diversity is normally limited to a version called “selection” diversity. In this case, as shown in FIG. 2, one antenna is used at a time. In FIG. 2, a first antenna 210 and a second antenna 212 each receive radio signals, including the radio signals from the same source but perhaps different paths. One of the two received radio signals is selected by a selection switch 220 for input to a Radio Frequency (RF) processor 230. The RF processor 230 down converts the received signal and performs other processing before the processed signal reaches a base-band processor 240. The base band processor 240 controls the operation of the RF processor 230. The base band processor 240 also provides a control signal to the selection switch 220 for controlling which antenna signal to process based on criterion such as measured signal strength. This selection as to which antenna used is based on criteria such as, for example, a measurement of the receive signal strength just before receive slot in a TDMA system.
On one hand, this version of antenna diversity does not fully exploit all that can be gained from the performance of combining signals from these two antennas. On the other hand, it avoids the cost in terms of extra components and power consumption, resulting from full processing of both antenna signals.
In International Patent Application WO 95/11552, a diversity receiver having two receiver branches is shown. A control signal based on comparison between signal strengths of the received signals is generated. Switching means are arranged to change-over a signal from either one of the receiver branches to a receiver output in dependence of the control signal. Hence, this is an example of a “selective” diversity system.
In European Patent Application 0,454,585, a diversity selection system switches between diversity antennas for every assigned time slot in a TDMA system to provide better signal quality according to a prediction algorithm. Hence, this patent is yet another example of a selective diversity system.
New systems for example IMT 2000 are being defined in a way where diversity is more or less necessary to meet some of the specific performance requirements. The overall criteria is to meet some kind of performance measure for the communication link. This can be monitored in any one of several ways. For example, one can measure the signal-to-noise ratio (SNR) of the received signal, one can measure or estimate a bit error rate (BER) or frame error rate (FER), or one can keep track of the number of required re-transmissions in cases where the radio link protocol uses re-transmissions. In most cases, this will require more complete implementation of diversity than selection diversity currently offers. Specifically, both antenna signals will need to be fully processed and the process signals combined in the most beneficial manner.
FIG. 3 is an example of such a system. In FIG. 3, a first antenna 310 receives radio signals and provides its received signals to a first RF processor 330. A second antenna 312 receives radio signals and provides its received signals to a second RF processor 332. The first RF processor 330 processes, e.g., down converts, the received RF signal to an intermediate signal for input to a base band processor 340. The first RF processor 330 also processes signals to be transmitted, including up converting the signal to a radio frequency for transmission over the first antenna 330.
Simultaneous with the processing of the first RF signal in the first RF processor 330, the second antenna 312 receives the same signal from the same source, but perhaps over a different radio path. This second signal is processed (e.g., down converted) in the second RF processor 332. The second signal is processed in the second signal processor 332 before being forwarded to the base band processor 340. The second RF processor 332 operates only in a receive mode.
The need for a lot of extra processing when using diversity is particularly true for systems based on CDMA, since in this case the antenna signal has to pass through most of the base band processing before the quality thereof can be judged. Simple measurements like signal strength do not give enough information about one individual user signal.
Although antenna diversity may be needed in some situations, there will be other cases where the radio environment allows perfectly adequate performance without diversity. If diversity is implemented such as shown in FIG. 3, the mobile station ends up spending power on the diversity processing whether necessary or not. This leads to shorter operating time for battery operated devices that could have been achieved without the power consumption in the diversity chain.